The present invention relates generally to a fuel injection control system for an internal combustion engine which temporarily cuts off the fuel supply under negligible engine load conditions. More specifically, the invention relates to fuel injection timing control upon resumption of the fuel supply after temporary fuel cut-off.
Japanese Patent First Publications Nos. 59-49339, 59-51137 and 59-90728, respectively published on Mar. 21, 1984, Mar. 24, 1984 and May 25, 1984 disclose acceleration enrichment during engine speed acceleration by way of asynchronous fuel injection. In such fuel injection control systems, the fuel injection amount for acceleration enrichment is so derived that the air/fuel mixture does not change abruptly.
On the other hand, Japanese Patent First Publication No. 58-163740, published on Sept. 28, 1983 discloses a fuel injection system which performs asynchronous fuel injection for fuel recovery after temporary fuel cut-off. In the shown system, the fuel injection amount in the asynchronous injection and subsequent synchronous injection are so derived that the total amount of the injected fuel in a asynchronous injection and in the immediately subsequent synchronous injection covers the fuel demand for engine speed recovery after fuel cut-off.
In this system, fuel injection amount during asynchronous injection alone is not sufficient for fuel supply resumption. Therefore, in a period between the asynchronous injection and the subsequent synchronous injection, the air/fuel mixture will become too lean to allow combustion in the engine cylinder. As a result, response to fuel supply resumption may be delayed by as much as 120.degree.. This clearly degrades engine performance and may even result in engine stalling if the engine speed should change too abruptly, as may occur during abrupt deceleration of the vehicle.
One possible solution would be to increase the fuel injection amount for asynchronous injection during fuel supply resumption. However, in this case, the air/fuel mixture would tend to be too rich in the subsequent synchronous injection. This would result in unnecessary increases in fuel consumption, and would be generally undesirable in view of air pollution by exhaust.